A need has long existed for an effective boat propeller that resists entanglement with weeds and other underwater plant growth. This need is particularly acute for fishing boats that utilize shallow lakes and waterways. Typically, such vessels employ relatively low speed trolling motors that permit them to be slowly but deliberately maneuvered while the occupants of the boat continue to fish. The conventional propellers employed by such motors have a strong tendency to become wrapped with weeds, particularly near the root of each propeller blade, which is the slowest moving portion of the blade. This can cause a significant loss of power and if severe enough can cause the motor to stall. At a minimum, the boater then incurs the delay and inconvenience of having to raise the motor and untangle the weeds therefrom.
Various attempts have been made to achieve weedless propeller operation. Weeds can be effectively cut through and pushed aside if a high speed motor is employed. However, such motors are likely to frighten fish and do not allow for slow paced trolling movement. Therefore, they are generally undesirable for use in many fishing boats.
Alternatively, cages have been mounted around the propeller. Unfortunately, such devices are themselves easily clogged by weeds. As a result, the propeller cannot generate sufficient thrust to properly power the boat.
As described in Hannon, U.S. Pat. No. 4,482,298, the configuration of the propeller has been specifically constructed in an attempt to provide weedless operation. In particular, the device of that patent employs an elongate hub that purports to provide an axial flow velocity that is greater than the tangential velocity of the hub. As a result, the residence time of the weeds about the hub is supposedly reduced so that entanglement is minimized. Notwithstanding the above, the Hannon device is still prone to entanglement, particularly at very low speed operations. Each of the blades of that device employs a curved leading edge having an arc or sweep that is fairly steep. As used herein, "blade arc" refers to the rate of positional change of the leading edge of the blade radially from the root to the tip relative to the positional change of the blade circumferentially from the leading edge to the trailing edge along an arc that is concentric with the hub. This arc may be defined by the formula dy/dx wherein dy denotes the radial change and dx denotes the circumferential change in the position of the blade's leading edge. At any given point along the leading edge the blade arc is determined by the slope of the line tangent to that point. In particular, in Hannon as well as in all other conventional propeller blades, the arc dy/dx&gt;1 along at least a portion of the leading edge. When such a blade arc encounters weeds it generates considerable friction, which is likely to cause the weeds to entangle the blades. This problem is exacerbated because the steep blade arc is located proximate the root of the blade, which is already slowest moving portion of the blade. Again, weed entanglement may be reduced by maintaining a high speed and the Hannon propeller is, in fact, designed primarily for use with a relatively high speed motor. However, that device does not exhibit optimally efficient weedless performance at very low trolling motor speeds on the order of 1100 rpm or less.